The polymerization of olefinic compounds and particularly lower .alpha.-olefins such as ethylene and propylene has gained substantial commercial acceptance. The polymer products thereby produced are relatively inexpensive and demonstrate commercially attractive properties. Numerous patents and literature references describe such polymerization processes and the practice of olefin polymerization on a commercial basis is widespread.
In the practice of olefin polymerization, reaction takes place under polymerization conditions that are now largely conventional and in the presence of an olefin polymerization catalyst. A variety of olefin polymerization catalysts have been employed with most being based on a transition metal. Recent polymerization catalysts are generally based on titanium and many if not most additionally contain moieties of magnesium and halogen. A number of polymerization processes incorporate molecular hydrogen in the reactor which serves to control the molecular weight of the polymer product as well as the product properties which correlate with molecular weight.
During commercial olefin polymerization it is beneficial to operate the polymerization process in an ongoing or minimally interrupted manner in order to minimize the down time of the polymerization line. Yet, most polymerization lines are designed to and in fact do produce product of differing types, whether because of differing chemical composition, differing properties or differing molecular weight. A change of reactant feed or reaction conditions is often difficult to accomplish without undesirable economic effects. The change from one monomer feed to another in an ongoing process typically results in the production of considerable "off-spec" polymer as, for example, a portion of the first monomer is found in the product produced from the newly introduced monomer feed, particularly if the first monomer is relatively inactive as compared with the monomers of the new feed. Such mixed or intermediate material is undesirable and must often be disposed of to the economic detriment of the process.
Whenever a change of product type is desired through feed changes or even when mechanical changes are necessary it is customary to temporarily retard or even stop polymerization so that the change can be made without the production of so much undesirable product. However, once the change has been made it is highly beneficial to resume polymerization as soon as possible to minimize down-time. One method of halting polymerization is through polymerization catalyst kill as by introducing to the polymerization reactor a catalyst poison such as carbon monoxide or certain esters, e.g., ethyl p-ethoxybenzoate. This procedure, although commercially employed, is limited to certain catalysts and has substantial detriment because the catalyst poison must be substantially completely removed from the polymerization reactor before resumption of the polymerization process. This removal is costly in terms of time and material. The effect of certain catalyst poisons can be overcome by the addition of an excess of the organoaluminum compound normally employed as the polymerization cocatalyst, but the provision of this excess is also expensive and leads to higher metallic residues in the product. It is also known that production of off-spec material through polymerization termination is accomplished by venting unreacted monomer and any other gaseous polymerization mixture components from the reactor. This venting of substantial quantities of unreacted monomer has an undesirable economic effect and also poses an environmental problem.
It is known that some few changes can be made in polymer product type without complete termination of polymerization. Burstain, U.S. Pat. No. 4,851,488, discloses a method for reducing the melt flow of a polypropylene polymer produced in an ongoing process by adding a hydrogenation catalyst to the polymerization reactor and hydrogenating a portion of the olefin monomer present. This serves to effectively lower the molecular hydrogen concentration within the polymerization reactor and, as a result, the melt flow of the polypropylene polymer thereby produced. The polymerization process is not completely terminated by this process although the hydrogenation catalyst must be removed or inactivated before normal polymerization can resume. Moreover, this particular process change is limited to reduction of melt flow or related properties. It would be of advantage to provide an improved method of temporarily terminating polymerization during an ongoing polymerization process to provide for mechanical, chemical or physical changes to the process and/or product types. Such method should permit rapid resumption of polymerization to polymer product of acceptable properties if uneconomical delays in process operation are to be avoided.